A Unix/Linux build won't be available for now, however, and the Norwegian browser maker was unable to confirm when that version would be spun out.

"As you may know, we don’t typically open to user-testing this early in the development cycle. However, we are really excited about what the Desktop team is cooking up and want your feedback," said Opera's Roberto Mateu in a blog post.

Opera is hoping to score kudos points from early testers who get to play with the outfit's JavaScript ECMAScript engine, which it has been dubbed Carakan.

Like Opera predecessors - including Futhark, Linear A, and Linear B - the engine shares its name with a much older breed of script. Carakan is both a modern-day JavaScript engine and pre-colonial Javanese script.

The plan is to eventually load that tech into the finalised version of Opera 10.5, said Mateu, who warned: "Please handle with care, backup your data before you install and do not run in hydroelectric power plants."

He claimed Carakan was a fast new JavaScript engine, which Opera built from the ground up after deciding to ditch the Futhark engine present in Opera 10.

Apart from the firm's major engine rejig, the pre-alpha build of Opera 10.5 comes loaded with improvements to the Presto layout engine, and a new graphics library named Vega.

The preview also nods in the direction of Firefox and Chrome with predictive smart search in the URL address bar.

Opera has tweaked the browser interface in version 10.5 by placing the tabs on top and minimising the menu bar. It's also been improved to play nice with Windows 7 and Snow Leopard platforms.